This invention relates generally to magnetoresistive sensors. More particularly, it relates to the stabilization of free layers"" magnetizations of magnetoresistive sensors.
Thin film magnetoresistive (MR) sensors or heads have been used in magnetic data storage devices for several years. Physically distinct forms of magnetoresistance such as anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR) and spin tunneling magnetoresistance (TMR) are well-known in the art. Magnetic read-back sensor designs have been built using these principles and other effects to produce devices capable of reading high density data. In particular, three general types of magnetic read heads or magnetic readback sensors have been developed: the anisotropic magnetoresistive (AMR) sensor, the giant magnetoresistive (GMR) sensor or GMR spin valve, and the magnetic tunnel junction (MTJ) sensor. The construction of these sensors is discussed in the literature, e.g., in U.S. Pat. No. 5,159,513 or U.S. Pat. No. 5,206,590.
For maximum spin-valve or tunnel-valve head stability and response linearity without hysteresis, it is generally desired, in the absence of any other source of external magnetic field on the free (or sensing) layer, that the magnetization of the free layer be maintained in a saturated single domain state. In such a state, the local magnetization everywhere in the free layer, up to and including the track-edges, will remain essentially xe2x80x9clongitudinalxe2x80x9d, i.e., co-linear with the cross-track direction of the head, parallel to the plane of the magnetic recording medium, and orthogonal to the direction of xe2x80x9ctransversexe2x80x9d magnetic signal fields emanating from a magnetic medium proximate the sensor.
The prior art has used a method of xe2x80x9chard-biasxe2x80x9d or edge-coupling-only to stabilize the magnetization of free (or sensing) layers of MR sensors. FIG. 1 illustrates the basic components of a typical current-in-plane (CIP) GMR sensor 100 with hard bias layers of the prior art. The sensor 100 includes a ferromagnetic reference layer 106 with a fixed transverse magnetic moment (pointing into the page) and a ferromagnetic free layer 110 with a rotatable magnetization vector, which can rotate about the longitudinal direction in response to transverse magnetic signal fields. The direction of the magnetic moment of the reference layer 106 is typically fixed by exchange coupling with an antiferromagnetic layer 104. Exchange-pinned reference layer 106 and free layer 110 are separated by a thin electrically conductive nonmagnetic layer 108. Hard bias layers 112 provide a longitudinal biasing magnetic field to stabilize the magnetization of the free layer 110 approximately in a longitudinal orientation in the absence of other external magnetic fields. Sensor 100 further includes top electrical leads 114 in proximity with hard bias layers 112, and a layer 102 adjacent to the antiferromagnetic layer 104, which represents a combination of the substrate, undercoat, and seed layers. For a shielded sensor, layer 102 may additionally include the bottom shield and insulation layers (for CIP sensors) or electrical contact layers (for CPP sensors).
FIG. 2 shows a current-perpendicular-to-plane (CPP) sensor 200 with hard bias layers of the prior art. CPP sensor 200 includes a ferromagnetic reference layer 206 with a fixed magnetic moment oriented transversely (into the page) and a ferromagnetic free layer 210 with a rotatable magnetization vector, which can rotate about the longitudinal direction in response to transverse magnetic signal fields. The direction of the magnetic moment of the reference layer 206 is typically fixed by exchange coupling with an antiferromagnetic layer 204. The exchange-pinned reference layer 206 and free layer 210 are spaced apart by a non-magnetic layer 208. For MTJ devices, layer 208 includes an electrically insulating tunnel barrier layer. For CPP-GMR devices, layer 208 is electrically conductive, and is analogous to layer 108 of the CIP-GMR sensor of FIG. 1. Hard bias layers 212 are electrically insulated from the sensor stack and the top electrical lead 216 by insulating layers 214 and 218 respectively. Hard bias layers 212 provide a longitudinal biasing magnetic field to stabilize the magnetization of the free layer 210. Sensor 200 further includes a layer 202, which is similar to layer 102 of sensor 100, in proximity with the antiferromagnetic layer 204.
An important concern in the design of the sensors of FIGS. 1 and 2 is the longitudinal bias of the free layers. It is desired that the hard bias layers maintain the free layer""s magnetization in a longitudinally oriented, single domain state. In the absence of longitudinal bias, the magnetization of free layer tends to establish a multi-domain state, as is well-known. Free layers in multi-domain states may experience Barkhausen jumps and other domain reorientation phenomena when responding to external magnetic fields from the encoding data bit in a magnetic recording disk. This problem is also known in the art and is highly undesirable as it produces hysteresis noise and worsens the signal-to-noise ratio (SNR) of the sensor.
However, the most common technique of the prior art includes the fabrication of magnetically hard (permanent magnet) bias layers which form an abutted junction with the physical track edges of the GMR sensor. For a CPP sensor, there exists the additional complication of maintaining an insulating spacer layer between the junction of the hard bias layers and the CPP stack. The efficacy of the method of stabilization depends critically on the precise details of the junction geometry, which is difficult to accurately control using present fabrication methods. The main source of this fabrication difficulty is the necessity of depositing and defining the hard-bias junction after the track width of the MR sensor is defined and patterned lithographically, and hence is subjected to the known fabrication and dimensional tolerances associated with this process.
An intrinsic consequence of any form of single domain stabilization of the free layer is the associated magnetic xe2x80x9cstiffnessxe2x80x9d of the free layer, which limits its rotational response to the magnetic signal fields from the recorded bits on the magnetic recording medium. For hard-bias, the stabilization mechanism is magnetostatic coupling to the free layer predominantly at or near the track edges proximate to the hard-bias junction. For edge-coupling-only stabilization in general, the average magnetic stiffness can progressively increase as read track widths shrink and track edges become relatively more proximate, and hence more tightly coupled, to the entire volume of the read head. The stiffness issue will be further exacerbated via the practical necessity of xe2x80x9cover-biasxe2x80x9d, in which the magnetic moment ratio (MS*t)bias/(Ms*t)free of the deposited hard-bias layer to that of free layer is designed to be several times greater than the theoretical minimum in order to compensate for the non-idealities (e.g., low coercivity) and geometric fabrication tolerances of actual hard-bias junctions. Further, the degree of required xe2x80x9cover biasxe2x80x9d is governed by the aforementioned fabrication tolerances, which are hard to control and hence difficult to design for.
U.S. Pat. No. 6,023,395 issued Feb. 8, 2000 to Dill et al. discloses a MTJ sensor, which includes, in addition to the necessary multitude of magnetic and nonmagnetic layers comprising a basic MTJ device known in the art, an extra in-stack ferromagnetic xe2x80x9cbiasingxe2x80x9d layer which is coupled exclusively magnetostatically with the ferromagnetic free layer. When the magnetization of the bias layer is essentially rigidly maintained with approximately longitudinal orientation, the disclosed purpose of the biasing layer is to provide some degree of longitudinal stabilization of the free layer. The form of in-stack stabilization described by Dill avoids some of the fabrication difficulties inherent to conventional hard-bias stabilization. However, the form in-stack stabilization described by Dill will suffer from the previously described read head sensitivity limitations of edge-coupling-only stabilization schemes if single domain stability is achieved. For example, it can be shown that the aforementioned xe2x80x9cover-biasxe2x80x9d issue of (Ms*t)bias/(Ms*t)free greater than 1 is a theoretical requirement for achieving single-domain stability using an in-stack stabilization geometry relying solely on magnetostatic (edge) coupling, such as described by Gill. If as disclosed by Gill, the ferromagnetic xe2x80x9cbiasingxe2x80x9d layer includes an exchange-pinned soft ferromagnetic layer, the high moment (Ms*t)bias of the biasing layer exacerbates the difficulty in achieving adequate exchange coupling between pinning and bias layers in order that the latter be maintained with magnetization rigidly aligned longitudinally.
There is a need, therefore, for an improved MR sensor having a free layer magnetization stabilized in a single-domain-state, which minimizes and/or accurately controls the trade-off between the stability and the sensitivity, and which has a favorable track-width scaling.
Accordingly, it is a primary object of the present invention to provide an MR sensor having a free layer""s magnetization maintained in a saturated single-domain-state in the absence of any external magnetic field.
It is a further object of the invention to provide a MR sensor with in-stack stabilization that minimizes the trade-off adjustability between the stability and sensitivity.
It is an additional object of the invention to provide a MR sensor having favorable track-width scaling.
It is another object of the present invention to provide a MR sensor with in-stack stabilization, which is manufacturable.
These objects and advantages are attained by MR sensor having in-stack single-domain stabilization of ferromagnetic free layers.
According to a first embodiment of the present invention, a MR sensor, such as a CIP-GMR sensor or a CPP-GMR/MTJ sensor, includes a ferromagnetic reference layer with fixed (pinned) magnetization, a ferromagnetic free layer and a first non-magnetic spacer layer disposed between the ferromagnetic reference layer and the ferromagnetic free layer such that the ferromagnetic free layer""s magnetization may rotate freely and independently of the magnetization of the pinned ferromagnetic reference layer. The MR sensor also includes a second non-magnetic spacer layer proximate to, and disposed between, the ferromagnetic free layer and a first auxiliary ferromagnetic layer, along with a second exchange-pinning layer for pinning the direction of orientation of the magnetization of the first auxiliary ferromagnetic layer. The MR sensor further includes a first exchange-pinning layer adjacent to the ferromagnetic reference layer for exchange-pinning the magnetization of the ferromagnetic reference layer. Alternatively, the MR sensor may include a ferromagnetic xe2x80x9ckeeperxe2x80x9d layer adjacent to the first exchange-pinning layer, and a third non-magnetic spacer layer disposed between the ferromagnetic xe2x80x9ckeeperxe2x80x9d layer and the ferromagnetic reference layer. The pinning directions set by the first and second exchange-pinning layers are nominally orthogonal, the first exchange-pinning layer being designated to fix the orientation of the reference layer in a xe2x80x9ctransversexe2x80x9d direction, and the second exchange-pinning layer being designated to pin the orientation of the first auxiliary ferromagnetic layer in a xe2x80x9clongitudinalxe2x80x9d direction. The MR sensor optionally includes a second auxiliary ferromagnetic layer adjacent to the second exchange-pinning layer, such that the magnetization of the second auxiliary ferromagnetic layer is pinned parallel to that of the first auxiliary ferromagnetic layer.
The second non-magnetic spacer layer is made of a material, such as Ru, which induces an anti-ferromagnetic RKKY-like exchange coupling between the ferromagnetic free layer and the first auxiliary ferromagnetic layer, promoting anti-parallel (AP) alignment between the magnetizations of these two layers. The third non-magnetic spacer layer also includes a similar (though not necessarily identical) AP coupling layer to promote anti-parallel alignment of the magnetizations of the ferromagnetic reference and xe2x80x9ckeeperxe2x80x9d layers.
The exchange-pinning layers are typically made of an antiferromagnetic (AF) material, or alternatively, a ferrimagnetic material. One or both of the first and second exchange-pinning layers also may be made of a non-conductive material to reduce the shunt loss of the sensed current in a current-in-plane (CIP) GMR sensor design.
The ferromagnetic free layer is longitudinally stabilized through a combination of AP exchange coupling and magnetostatic coupling between the nominally anti-parallel ferromagnetic free layer and the first auxiliary ferromagnetic layer. The combined approach of exchange plus magnetostatic stabilization minimizes; the respective drawbacks that may occur if either stabilization method is used exclusively. Furthermore, the in-stack stabilization structure maintains a more favorable and adjustable track-width scaling as track-width is reduced. Fabrication of MR sensors with in-stack stabilization structure is more readably manufacturable since all the sensor layers and bias layers are deposited in-stack.
MR sensors of the type described in the first embodiment may be incorporated into MR sensing heads according to a second embodiment of the present invention. A MR sensing head includes a first shield proximate a first gap, a second shield proximate a second gap, and a MR sensor disposed between the first and the second gaps.
MR sensing heads of the type depicted in the second embodiment may be incorporated into a disk drive according to a third embodiment of the present invention. A disk drive includes a magnetic recording disk connected to a motor and a MR sensing head connected to an actuator. The motor spins the magnetic recording disk with respect to the MR read/write head, and the actuator positions the MR sensing head relative to the magnetic recording disk.